1. Field of the Invention
This invention relates generally to methods for determining the calibration, size and number of generators to produce predetermined levels of radioisotopes and, more particularly, to methods to provide a preferred generator profile of the generators required to meet a predetermined requirement of radioisotopes.
2. Description of the Related Art
In recent years, diagnostic nuclear medicine has proven to be of enormous value to the medical community. Procedures for imaging and detecting abnormalities in the brain, liver, lungs, bones, and the like have been well developed and are routinely used. These procedures are based on the tendency of the body to concentrate some chemical form of a particular gamma ray emitting isotope in the organ of interest. Subsequent scanning of the organ with a gamma ray camera provides an image of the organ from which diagnostic information can be obtained.
It has long been known that the introduction into an organism of compounds containing (or "labeled" with) a radioisotope can provide insight into the anatomy and physiology of the organism. These compounds, generally referred to as radiopharmaceuticals, are particularly useful in diagnostic techniques which involve studying the structure or function of various internal organs, e.g., the brain, with radiation detection means. For diagnostic work, isotopes with a short half life and an emission spectrum rich in gamma rays (as opposed to beta particles) are preferred. It is clear that the radioisotope with optimum nuclear properties (half-life, gamma ray energy, and the like) for medical gamma ray scanning is .sup.99m Tc, or "Tc-99m."
The metastable isotope Tc-99m has a 6 hour half-life and an emission spectrum of 99% gamma radiation at 140 KeV, which is well suited for techniques of diagnostic nuclear medicine. Tc-99m has a high specific activity, 5.28.times.109 millicuries per gram, and a convenient rapid rate of decay. For the researcher or clinician, the emission spectrum of Tc-99m can provide high levels of accuracy in radiodiagnostic measurements and calculations. In recent years, Tc-99m has become readily available in hospitals through the use of selective elution or removal of that radioisotope from a so-called molybdenum-99 (Mo-99) generator.
The isotope Mo-99 produces Tc-99m as a radioactive decay product and Tc-99m can be removed from a Mo-99 generator by eluting the generator. After an elution is made, the generator will start to regenerate Tc-99m and then can be re-eluted in a few hours when minimum levels of Tc-99m have been generated. However, the amount of Tc-99m obtained from an elution depends on several factors, including the amount of Mo-99in the generator, the amount of time elapsed since the generator was last eluted, and variable factors in the eluting environment that influence elution efficiency.
A generator is described by several parameters, including the manufacturer of the generator and its size and calibration. The size of a generator is not the physical size, but rather the amount of Mo-99, expressed in terms of millicuries (mCi), in the generator. The calibration of a generator is the day of the week that the generator contains the labelled activity. For example, a Monday calibrated 1800 mCi generator will contain 1800 mCi on Monday and a Thursday calibrated 1800 mCi generator will contain 1800 mCi on Thursday. These two generators are labelled as the same size, 1800 mCi, and will cost the same, yet they will produce greatly different Tc-99m yields on a given day. The day of the week a generator is received is also a parameter for its identification. The earlier it is received after calibration, the more activity a generator will have.
The short half-life of Tc-99m (six hours) significantly decreases a pharmacy's ability to store it. The half-life of a Mo-99generator is 3 days, which is a more reasonable amount of time. However, meeting weekly requirements of Tc-99m requires significant guess work and estimation as to the level of Tc-99 available in a Mo-99 generator at a given time. In the past, scheduling of deliveries of generators has been based upon experience. This often resulted in an inaccurate determination of a generator profile, that is, the specification of generators, in terms of the manufacturer, size, calibration and date of delivery, for a preselected period of time. Accordingly, there is a need for a method to provide a preferred generator profile, by size and calibration, that will meet a pharmacy's requirements of radioisotopes, while avoiding undue wastage of radioisotopes. The present invention fulfills this need.